This invention relates generally to the field of explosive shock wave attenuators, especially to attenuators designed to be inserted between mass-detonable explosives to prevent the propagation of accidental explosions by sympathetic detonation of adjacent explosives, and more particularly to attenuators suitable for use in the close environment of a logistic missile container, or inside a missile between the warhead and rocket motor.
The use of attenuating materials between mass-detonable explosives such as projectiles, bombs and missile propellants is well known. The goal has been to reduce the risk of an accidental explosion of one explosive from spreading by sympathetic detonation of adjacent explosives. Obtaining this goal reduces the spacing normally required for safe storage of such devices, creating savings in both space and siting costs. Explosives and propellants would be safer to store, transport and handle. A more efficient attenuator will help gain safety acceptance for the use of hazard Class/division 1.1, or min-smoke, rocket motors in place of the less powerful Class 1.3 rocket motors now generally used, and will make existing Class 1.1 warheads safer to handle.
Attenuating material used between mass-detonable explosives is typically sacrificial, in that a substantial portion of the explosive energy to be absorbed by the attenuator is dissipated in crushing or otherwise deforming the attenuating material. Typical sacrificial attenuator materials used in the past are earth, foamed concrete, layered wallboard, or steel I-beams. These materials are thick and heavy and are unsuitable for use in close environments such as logistical containers for the storage of missiles, or inside the missiles to separate the explosives contained in the warheads from the explosive propellants contained in the rocket motors. Thinner, and also lighter, attenuators are needed.
One proposed solution to the need for a better attenuator for this use has been perforated plates, a thinner variation of typically bulky baffled-venting methods. The perforated plates attenuate by the rapid dissipation of the energy required to force jets of air or other gases through the openings in the plates. Although relatively light in weight, the perforated plates have had problems of projecting secondary fragments in an explosion. Pairs of perforated plates have been tested with apparently better results and would be suitable where wider spacing between missiles is available.
Another proposed solution has been the use of sacrificial rigid foams such as scoria, a foamed glass of volcanic origin. These rigid foams, when shaped to meet the requirements of typical logistical missile containers, will not survive the rough handling and other requirements of those containers.
The use of laminates to attenuate the propagation of projectiles, shock vibration from explosions, and the shrapnel that often accompanies explosions, is well established. Laminates are made generally either to combine the desired properties of two or more materials, or to take advantage of the consecutive reflections of the shock wave that takes place at the interfaces between the materials forming the laminations. These consecutive reflections increase the time and distance for the entire energy of an incident shock wave to pass through the material, both spreading out the wavefront, and increasing the attenuation through conversion to heat from internal friction. The resistance of a material to the transmission of vibration is termed acoustic impedance. Most of the laminates used to date have consisted of laminations of material of alternating acoustic impedances, while the literature has recommended the use of laminations of successively reduced acoustic impedances to take advantage of the increased attenuation of the peak stress of a vibration wavefront that occurs when vibration crosses consecutive interfaces from materials of higher to lower acoustic impedance.
Polyaramid filaments, such as Kevlar.TM., when mixed with a resin to form sheets or plies, have seen increasing use as an attenuator material, especially against the propagation of projectiles.
Despite the variety of approaches which have been tried in the past, the prior art does not disclose an optimum combination of attenuator material and design for use between mass-detonable explosives, particularly a design specifically suitable for the close environments found in missile storage containers and inside missiles, and where transportation by air requires minimizing dead weight.
With the foregoing in mind, it is, therefore, a principal object of the present invention to provide an improved attenuator suitable for use between explosives where the direction from which the initial accidental explosion will occur is unknown, and which incorporates protection against sympathetic detonation in a more efficient, and thus thinner and lighter, structure.